The present invention relates to a precursor for producing an Nb.sub.3 Sn compound superconducting wire used for a high magnetic field superconducting magnet, a method for producing the precursor, and a method for producing an Nb.sub.3 Sn compound superconducting wire.
FIG. 9 is a sectional view of a precursor for producing an Nb.sub.3 Sn compound superconducting wire by a conventional internal tin diffusion method, and FIG. 10 is a sectional view of a compound superconducting wire produced from the precursor by heat treatment. For example, the precursor and the compound superconducting wire are disclosed in JP-A-57-82911.
In FIG. 9, the reference numeral 17 designates an Nb.sub.3 Sn compound superconducting wire precursor before heat treatment (hereinafter referred to as "precursor"). The precursor 17 is composed of filaments 18 of a niobium(Nb)-base metal which will be made superconductive by heat treatment, a matrix 19 of a copper(Cu)-base metal for embedding the filaments 18, a barrier material 5 of tantalum (Ta) provided on the outer circumference of the matrix 19, a stabilizing material 6 of oxygen-free copper provided on the outer circumference of the barrier material 5, and a tin-base core 20 of an Sn-2% Ti alloy material embedded in the center portion of the matrix 19.
In FIG. 10, the reference numeral 21 designates an Nb.sub.3 Sn compound superconducting wire after heat treatment (hereinafter referred to as "compound superconducting wire"). The compound superconducting wire 21 is composed of superconducting filaments 22 of Nb.sub.3 Sn produced by heat treatment, a matrix 23 of a Cu-base metal for embedding the superconducting filaments 22, a barrier material 5 provided on the outer circumference of the matrix 23, and a stabilizing material 6 of oxygen-free copper provided on the outer circumference of the barrier material 5. The matrix 23 is provided as low-concentration Sn bronze because Sn in the tin-base core 20 is diffused at the time of heat treatment.
The precursor 17 shown in FIG. 9 is produced as follows.
First, an Nb rod is inserted in a Cu pipe and the section of the Cu pipe is reduced to a predetermined size, so that a filament material of Cu-coated Nb wire is formed. The filament material is cut into a suitable length to form a large number of filament materials. A billet of Cu is filled with the large number of filament materials. A rod of Cu is arranged or a large number of Cu wires are arranged in advance in the center portion of the billet. The billet is evacuated, sealed with a cover, and then subjected to extruding. Then, a hole is mechanically formed in the center of the billet to form a hollow portion. A tin-base core material of Sn-2% Ti alloy is inserted in the hollow portion. The outside of the billet subjected to extruding is coated with a Ta pipe and with a Cu pipe successively. Further, the section of the whole is reduced to a predetermined size. Thus, a precursor 17 shown in FIG. 9 is produced. Incidentally, in order to make the current capacity high, the section of a Cu pipe filled with a large member of such precursors 17 may be reduced.
The precursor 17 produced as described above is twisted, and then subjected to preheat treatment and final heat treatment (generally, at a temperature in a range of from 600.degree. C. to 800.degree. C.) to thereby obtain the compound superconducting wire 21 shown in FIG. 10.
By the final heat treatment, Sn in the tin-base core 20 of Sn-2% Ti alloy in the precursor 17 shown in FIG. 9 is diffused into the ambient matrix material 19 to change the matrix 19 into a Cu-Sn alloy and, further, Sn reacts with the filaments 18 to generate Nb.sub.3 Sn in the surfaces of the filaments 18 or in all the filaments 18. Thus, the superconducting filaments 22 shown in FIG. 10 are produced.
The compound superconducting wire 21 according to the internal tin diffusion method as shown in FIG. 10 has a structure in which superconducting filaments 22 of Nb.sub.3 Sn generated by heat treatment are embedded in the matrix 23 as densely as possible while being prevented from being in contact with one another in order to increase as large as possible, the critical current density (Jc) which is one of superconducting properties.
Further, in order to improve the Jc property in a high magnetic field through improvement of an upper critical magnetic field which is one of the superconducting properties, Ti is added to the superconducting filaments 22 of Nb.sub.3 Sn. There are various methods for adding Ti as follows.
In an internal tin diffusion method, employed are a method of adding Ti as an alloy to a tin-base core 20 as shown in FIG. 9 (JP-A-62-174354), a method of adding Ti as an alloy to filaments 18 shown in FIG. 9 (JP-A-60-170113), and a method in which both the two methods mentioned above are used in combination.
In a so-called bronze method using a precursor which is configured such that an Nb-base metal material is embedded in the matrix 19 provided as a Cu-Sn alloy, employed are a method of adding Ti as an alloy to filaments 18 (JP-A-57-54260), and a method of adding Ti as an alloy to the matrix 19 (JP-A-58-23110).
In a so-called jelly roll method using a precursor which is configured such that rolls of Nb foil used instead of the Nb rods are embedded in the matrix 19, employed is a method of adding Ti as an alloy to the Nb foil (PCT Application: PCT/US 90/054/08).
The methods of adding Ti as an alloy in the conventional internal tin diffusion method, bronze method and jerry roll method have the following problems (1) to (6) in production and use of the alloy.
(1) It is difficult to produce a Ti-added alloy because of generation of a Ti intermetallic compound or work-hardening. Accordingly, a good-quality alloy material free from breaking cannot be obtained. PA1 (2) When any other metal such as Mn, etc. than Ti is added simultaneously with Ti, an intermetallic compound is generated to make it difficult to process a Ti-added alloy. PA1 (3) In production (vacuum melting) of a Ti-added alloy, oxygen impurities such as Ti oxide, etc. increase because the vapor pressure of Ti is so high that the degree of vacuum at the time of vacuum melting cannot be increased. Accordingly, the superconducting property of the superconducting filaments 22 is worsened by the oxygen impurities. PA1 (4) In production of a Ti-added Sn alloy, the size of the Ti intermetallic compound varies in accordance with the cooling speed. Accordingly, when the size of the Ti intermetallic compound is large, Jc in the superconducting filaments 22 varies. PA1 (5) The cost for production of a Ti-added Nb alloy increases because vacuum melting is required. PA1 (6) In the internal tin diffusion method, the tin-base core 20 of Sn--Ti is embedded in the center portion of the matrix 19. Accordingly, in preheat treatment for diffusing Sn and Ti, the concentration gradient of Ti is generated between the inner and outer arrays of filaments 18. After final heat treatment, the outer array of filaments 18 are inferior in Jc property to the inner array of filaments 18 and lower in n-value which is one of the superconducting properties (the n-value is an index for indicating uniformity in the longitudinal direction of a superconducting wire, that is, the superconducting property becomes excellent as the n-value increases).